1. Field of the Invention
The present invention relates to a fixing unit, and more specifically, to a fixing unit including a roller provided with two tubular heaters.
2. Description of Related Art
A conventional type of fixing unit of an image forming apparatus, for example, as disclosed in Japanese Patent Application Laid-open No. H8-220930, includes a heat roller for supplying heat for fixing which stores two tubular heaters each composed of a halogen lamp. These two heaters may have respectively different heating areas so as to efficiently support different widths of recording paper. In a manufacturing process of a halogen lamp, during sealing of a silica glass tube, one protruding residual fused tip (tip portion) is inevitably formed on the outer circumferential surface of the silica glass tube. The shape of such a tip portion depends on the shape of a jig used for sealing the glass tube, but is typically substantially columnar (see FIG. 3). The specifications of a halogen lamp for a fixing unit define the height of a tip portion, for example, up to 3 mm maximum for a silica glass tube of 6 mm in diameter. Therefore, the halogen lamp typically has a tip portion of approximately 3 mm in height formed on the outer circumferential surface of a silica glass tube thereof.
Hereinafter, the heater structure and the heater fitting structure of a conventional fixing unit will be described, referring to FIGS. 22 and 23. Two heaters 102 each have flat terminals 124 respectively soldered to lead wires 123 that protrude from both ends of a silica glass tube 121. The flat terminals 124 are respectively fixed with screws to fitting surfaces 141 provided at both longitudinal end walls of a case, not shown, of the fixing unit. The specifications of a typical heater define the orientation of a tip portion 121a with respect to the flat terminal 124. For example, as viewed in side views of FIGS. 22 and 23, the height H direction of the tip portion 121a is parallel to the width W direction of the flat terminal 124 (see FIG. 22), while the height H direction of the tip portion 121a is tilted at 90 degrees with respect to the width W direction of the flat terminal 124 (see FIG. 23). Thus, there are two possible arrangements for the heaters 102. First, the heaters 102 are respectively fixed to the fitting surfaces 141 through the flat terminals 124 so that, as shown in FIG. 22, the tip portions 121a of the two heaters 102 are arranged in the same direction with the height direction H thereof parallel to a connection line C1 connecting the centers of the silica glass tubes 121. Alternatively, as shown in FIG. 23, the tip portions 121a of the two heaters 102 are arranged with the height direction H thereof orthogonal to the connection line C1 connecting the centers of the silica glass tubes 121 and also in line-symmetry to each other with respect to a center line C2 passing through the center of the heat roller 101.
Therefore, with the arrangement shown in FIG. 22, the two heaters 102 need to be spaced apart at some distance from each other (a distance L between their centers needs to be provided in a sufficient amount) and also the inner diameter of the heat roller 101 needs to be increased so that the tip portions 121a do not interfere with the silica glass tubes 121 arranged next thereto, respectively, or with the inner wall of the heat roller 101. Accordingly, the outer diameter D of the heat roller 101 increases, thus resulting in failure to satisfy the demand for space-saving design of parts to follow the recent downsizing trend of image forming apparatuses.
On the other hand, with the arrangement shown in FIG. 23, the heaters 102 can be arranged in some proximity to each other; however, the proximity between the heaters 102 causes a problem that the heaters 102 are likely to make contact with each other to be thereby broken upon their installation.